USE OF A MASS-THICKNESS MARKER TO ESTIMATE SYSTEMATIC-ERRORS AND STATISTICAL NOISE IN THE DETECTION OF PHOSPHORUS BY ELECTRON SPECTROSCOPICIMAGING

The element signal obtained from electron-energy-filtered micrographs depends on the systematic error in calculating the background and on t he noise in the background-corrected image. Both systematic error and statistical fluctuation of the background can be assessed experimental ly with a specimen that combines the element-containing feature with a mass-thickness marker. The approach is described for the mapping of p hosphorus in turnip yellow mosaic viruses prepared on a supporting car bon film of variable thickness. The thickness modulations are produced by the additional deposition of heat-evaporated carbon through a seco nd grid used as a mask. The three-window power-law method and the two- window difference method are compared. With the three-window power-law method, the mass-thickness modulations of the marker are still visibl e in the map, indicating a systematic error for the calculated backgro und. In addition, the intensity profile over the area of the thick car bon him is broader than in the map corrected by the two-window method, indicating a higher level of noise. With the two-window difference me thod, mass-thickness contrast was practically eliminated due to an imp roved protocol that uses the mass-thickness marker to calculate the sc aling factor: instead of scaling the grey-level of a single background feature, the pre-edge image is scaled to the contrast of the marker a rea in the image acquired at the element-specific energy loss. (C) 199 7 Elsevier Science Ltd. All rights reserved.